refactor: standardize --agent option usage, improve YAML registry fallback, and fix layout falsy-zero trap (J-1)

- In stop_session.sh and update_yaml_resumed.sh: standardize explicit --agent option and use resolve_agent_type_from_registry to read agent type from YAML/DB state rather than brittle suffix-only regex inference.
- Update multi-agent-mux-stop/SKILL.md, multi-agent-mux-resume/SKILL.md, multi-agent-mux-create/SKILL.md, and deploy/INSTALL.md to standardize passing --agent explicitly.
- Fix J-1 in layout.py: refactor _env_int(*names, default=None) to take an explicit default parameter, eliminating the falsy-zero trap so MAM_MIN_PANE_COLS=0 is respected.
- Add regression and contract tests: test_j1_env_zero_min_cols_matches_flag_zero, test_j1_env_zero_min_rows_matches_flag_zero, test_comp_stop_agent_fallback_*, test_comp_docs_stop_examples_pass_agent.
- Verified 100% UNANIMOUS PASS from Planner claude and Reviewers claude and cline.

tests/test_tier2_component.py

@@ -1,4 +1,5 @@
 import os
+import re
 import shutil
 import json
 import sqlite3
@@ -293,7 +294,7 @@ d['herdr_sessions'] = [{
 
 
 # ==============================================================================
-# FEATURE 3: Stop Session (4 Test Cases)
+# FEATURE 3: Stop Session (7 Test Cases)
 # ==============================================================================
 
 def test_comp_stop_sqlite_state_update(mam_sandbox):
@@ -411,6 +412,70 @@ d['herdr_sessions'] = [{
     assert any(("send" in call or "prompt" in call) and "/exit" in call for call in calls)
 
 
+def test_comp_stop_agent_fallback_reads_pane_cmd(mam_sandbox):
+    """B-21: --agent 생략 시 세션명에 에이전트 접미사가 없어도 레지스트리 행의
+    pane.cmd 로 해석된다 (라이브 `agy-creator-01` 형태)."""
+    mutation = """
+d['herdr_sessions'] = [{
+    'name': 'agy-creator-01',
+    'status': 'running',
+    'pane': {'cwd': 'WS_PLACEHOLDER', 'cmd': 'agy'}
+}]
+""".replace("WS_PLACEHOLDER", str(mam_sandbox))
+    run_mutation(mam_sandbox, mutation)
+    script = mam_sandbox / ".agents" / "skills" / "multi-agent-mux-stop" / "scripts" / "stop_session.sh"
+    res = subprocess.run(["bash", str(script), "--session", "agy-creator-01"],
+                         capture_output=True, text=True)
+    assert res.returncode == 0, res.stderr
+    assert re.search(r"^\s*agent:\s+agy\s*$", res.stdout, re.M), res.stdout
+
+
+def test_comp_stop_agent_fallback_prefers_explicit_agent_field(mam_sandbox):
+    """우선순위 계약: 명시 `agent` 필드가 세션명 접미사와 pane.cmd 를 모두 이긴다."""
+    mutation = """
+d['herdr_sessions'] = [{
+    'name': 'x-creator-claude',
+    'status': 'running',
+    'agent': 'hermes',
+    'pane': {'cwd': 'WS_PLACEHOLDER', 'cmd': 'claude'}
+}]
+""".replace("WS_PLACEHOLDER", str(mam_sandbox))
+    run_mutation(mam_sandbox, mutation)
+    script = mam_sandbox / ".agents" / "skills" / "multi-agent-mux-stop" / "scripts" / "stop_session.sh"
+    res = subprocess.run(["bash", str(script), "--session", "x-creator-claude"],
+                         capture_output=True, text=True)
+    assert res.returncode == 0, res.stderr
+    assert re.search(r"^\s*agent:\s+hermes\s*$", res.stdout, re.M), res.stdout
+
+
+# 코드 펜스 안의 stop_session.sh 호출을 '명령 단위'로 잘라낸다.
+#  - 펜스 스코프: 산문 속 `stop_session.sh` 언급을 명령으로 오인하지 않는다
+#    (Pitfalls / When-NOT-to-use 절은 성격상 스크립트를 산문으로 언급한다).
+#  - 명령 단위: 한 펜스에 여러 호출이 들어 있어도 각각을 따로 검증한다
+#    (블록 단위로 보면 그중 하나만 --agent 를 가져도 통과해 버린다).
+_FENCE_RE = re.compile(r"```(?:bash|sh)\n(.*?)```", re.S)
+_STOP_CALL_RE = re.compile(r"(?:bash\s+)?\S*stop_session\.sh[^\n\\]*(?:\\\n[^\n\\]*)*")
+
+
+def test_comp_docs_stop_examples_pass_agent():
+    """B-21 문서 계약: 문서의 모든 stop_session.sh 예제는 --agent 를 넘긴다.
+    문서 변경은 뮤테이션 감도가 없으므로 이 가드가 표준의 유일한 집행 장치다."""
+    repo = Path(__file__).resolve().parent.parent
+    expected = {  # 문서별 최소 예제 수 — 예제를 지워 가드를 무력화하는 것을 막는다
+        repo / ".agents/skills/multi-agent-mux-stop/SKILL.md": 3,
+        repo / "deploy/INSTALL.md": 2,
+    }
+    for doc, floor in expected.items():
+        seen = 0
+        for block in _FENCE_RE.findall(doc.read_text()):
+            for m in _STOP_CALL_RE.finditer(block):
+                snippet = m.group(0)
+                seen += 1
+                assert "--agent" in snippet, \
+                    f"{doc.name}: stop_session.sh example without --agent:\n{snippet}"
+        assert seen >= floor, f"{doc.name}: expected >= {floor} examples, saw {seen}"
+
+
 # ==============================================================================
 # FEATURE 4: Status Query (5 Test Cases)
 # ==============================================================================